Note: this site last updated in 2006
An article from "The Molecules of HIV" (c) Dan StowellbindingAndFusion.gif" />
HIV-1, once it has attached to a potential victim cell, needs to fuse its membrane with the cell's membrane. This is so that the HIV core - its RNA and some proteins - can be safely imported into the cell.
The docking of gp120 and CD4 causes conformational changes in HIV's envelope proteins (remember that gp120 and gp41 together are the envelope proteins) - i.e. their shape changes slightly - forcing the fusion area of gp41 to be exposed. (The fusion area is a hydrophobic area which embeds into the cell membrane to help fuse the two lipid bilayers.)
Other cell molecules apart from CD4 are required for membrane fusion. Fusin (CXCR4), a receptor which crosses the cell membrane 7 times, is necessary for HIV to enter T cells (though not for entry into macrophages). Other receptors (CCR3, CCR2b) can act as "coreceptors" which enhance viral entry for some viral isolates.
The pattern of amino acids in the V3 loop (the third variable loop) of gp120 determines whether fusin or R5">CCR5 will be the accessory protein usable by HIV for membrane fusion. Some versions of HIV therefore use fusin while some use R5">CCR5, and this can affect the type of cell targeted by each particular strain. (Some strains can tend to target macrophages rather than T cells.)
A side note: another way to get stuff out of the virus and into the cell would be for the virus to be able to "inject" its proteins/genes into the cell. Many bacteriophages (viruses which prey on bacteria) do this. Also, plenty of viruses exist without membranes: they only have to attach to the cell and be absorbed into it rather than try and merge membranes.